1. Field of the Invention
The present invention relates generally to the field of position detection and, more specifically, to an ultrasonic method and apparatus for detecting position of, for example, the movable internal parts of a check valve.
2. Description of the Related Art
Ultrasonic techniques have been used to effect non-destructive testing for part defects or fractures and to effect distance measurement and object detection.
In the nuclear industry, a great deal of attention is directed towards non-intrusive testing and on-line monitoring of, for example, check valves in nuclear power plants. Because these valves, those simple in function, are critical to safe plant operation, they are subject to Nuclear Regulatory Commission (NRC) specified periodic inspections that often require time consuming and expensive partial disassembly.
With particular reference to check valves (which are non-motorized), it is also desirable to not only detect an open (i.e., fully open) or closed position, but also various positions between open and closed positions, as well as the nature of valve element motion. A fluttering motion, for example, could indicate a condition causing excessive wear. Although completely non-intrusive monitoring of unmodified installed valves is desired, methods that require some modification are acceptable, particularly if monitoring capability is greatly enhanced. An example is a magnetic technique, whereby a permanent magnet is mounted on the disk assembly of a check valve and changes in the magnetic field are sensed externally. The fact that a linear change in field cannot be produced and magnetic techniques may not be applicable at all for carbon steel valves are limitations of these techniques. Also, the choice of materials for the modification is limited by the need for permanent magnetism.
While ultrasonic techniques have been employed for check valve inspection and testing, certain problems make their use difficult in this application. For example, valve bodies have contours which limit the possible acoustic path directions, and surface finish may not be adequate for good acoustic coupling. Differences among valves of various manufacturers exacerbate the problem.
While general principles of ultrasonic testing are disclosed in U.S. Pat. Nos. 4,069,433 and 3,925,692, a continuing need exists for an improved ultrasonic testing technique and apparatus for determining valve element position.